The overall objective of the project is to analyze in detail the biochemical processes involved in the synthesis and assembly of the contractile proteins during cardiac myogenesis. Recent studies in this and other laboratories have suggested that changes in polymorphic forms of several myofibrillar proteins take place during the transition from embryonic to adult stages of development of avian and mammalian skeletal muscles. We have postulated that the characteristics ontogenic changes in myosin light chains and tropomyosin subunits observed in skeletal muscle fibers are due to an intrinsic developmental program which operates in all fibers regardless of their adult fiber type. One important question in the area of cardiac myogenesis is whether some kind of developmental program, as has been proposed for skeletal muscle development, also operates in cardiac muscle during development and how this process is related to that observed in skeletal muscle. To answer this question I shall look into the myosin and tropomyosin subunits during cardiac muscle development. In parallel studies I shall examine changes in cellular mRNA populations coding for isozymes of myofibrillar proteins in order to determine whether the changes in protein pattern are regulated at the transcriptional and/or translational level. Finally, using the heteropolymeric myosin molecule as a model complex myofibrillar protein, I plan to study the synthesis and assembly of the protein from newly synthesized subunits using cell-free polysomal preparations from chick embryonic heart muscles. The proposed studies will hopefully elucidate the major biochemical events associated with normal cardiac muscle development - an area where many questions related to changes in myofibrillar protein patterns remain to be understood. They may also shed some light on the general problem of the regulation of gene expression during eukaryotic growth and differentiation.